This invention relates to the color stabilization of refined dicarboxylic acid anhydrides. More particularly, it relates to the color stabilization of refined maleic anhydride.
Anhydrides of certain dicarboxylic acids are available to the consumer in either solid or molten form. Although these anhydrides are classified as chemically stable compounds, some discoloration of the solid material may be detected after extended storage periods. This rate of discoloration is more pronounced when the solid anhydrides are maintained in their molten state for extended periods at elevated temperatures.
Maleic anhydride, for example, is often shipped in molten form in heated, insulated tank cars and is thus maintained in this state for long periods of time. Under such conditions, the molten maleic anhydride will frequently darken and become discolored.
Maleic anhydride can be prepared by the vapor-phase oxidation of organic compounds such as benzene, toluene, naphthalene, methyl naphthalene, phenol, cresol, benzophenone, furan, biphenyl, furfural, n-butane, 1-butene, 2-butene, butadiene, heptane, isooctane, crotonaldehyde and crotonic acid, employing a large excess of air. By-products of the above reaction include other organic acids, chromogenic bodies, carbon monoxide, carbon dioxide and water. Crude maleic anhydride is generally very dark in color, and although the crude maleic anhydride can be refined to a substantially color-free material, color reappears upon storage as hereinabove described. Color is an undesirable characteristic of maleic anhydride and, if present before or during processing, can cause deleterious effects in resulting products, such as plastics, where proper color is an important feature of the material.
The prior art discloses various methods for refining crude maleic anhydride to obtain a high grade product of low color. One such method is disclosed in U.S. Pat. No. 2,296,218 to Middleton, wherein crude maleic anhydride containing volatile color-imparting compounds produced as byproducts in the partial oxidation of organic compounds is treated in the liquid state with a modification agent selected from the group consisting of the oxides and hydroxides of sodium, potassium, lithium, calcium, zinc and magnesium, and the halides of zinc, iron and aluminum and reaction products thereof. Thereafter, the treated maleic anhydride is subjected to distillation, leaving the darkening impurities in the distillation residue.
Another method for improving color stability is disclosed in U.S. Pat. No. 3,115,503 wherein color stabilizing amounts of ethylenediaminetetraacetic acid are added to a molten cyclic anhydride during the preparation process or thereafter. Alternatively, the acid is added to the finely divided solid cyclic anhydride prior to compressing into tablets or briquets.
Another method is taught in U.S. Pat. No. 3,596,703 wherein small amounts of a stabilizer are incorporated into maleic anhydride. The stabilizers used included hydrochloric acid, hydrobromic acid, silver chloride, barium nitrate, alkali or alkali metal sulfates, chlorides or bromides. Similar compounds were found to have no usefulness as stabilizers.